This invention relates to a low-pass filter for removing a high-frequency component from an input signal and, in particular, to a low-pass filter with an adding function.
In prior art, an echo canceller or suppressor is used as one means realizing a two-way communication and is standardized by the American National Standards Institute (ANSI) T1. 601 in the integrated services digital network (ISDN).
In the two-way communication, transmission and reception are simultaneously carried out. As a result, a receiver side receives not only a received signal but also an echo signal where a transmission signal is turned back from a transmitter side to the receiver side. The received signal, which is transmitted via a transmission line, decays up to 50 dB but the echo signal only decays by about 6 dB because almost all of the transmission signal is turned back via a line transformer as the echo signal. Accordingly, there is a chance that the echo signal has a higher lever of 40 dB (a hundred times) or more in comparison with the received signal.
It is therefore necessary to remove the echo signal from the received signal in the receiver side. As a result, the receiving side comprises an adder for removing the echo signal from the received signal and a low-pass filter for preventing turned-back noises in an added signal. In prior art, the adder and the low-pass filter constitute a combinational circuit having an adding function and a low-pass filtering function, individually. In other words, the combinational circuit comprises the adder and the low-pass filter. In the manner which will later be described in conjunction with FIG. 2, each of the adder and the low-pass filter is implemented by an operational amplifier and other peripheral passive elements. As a result, a conventional combinational circuit is disadvantageous in that it has a large scale or size in circuitry and dissipates a significant amount of power.
On the other hand, an active filter is disclosed in Japanese Unexamined Patent Publication of Tokkai No. Hei 1-321,716, namely, JP-A 1-321,716. The active filter according to JP-A 1-321,716 has a plurality of filter input terminals and a filter output terminal, and is implemented by only one operational amplifier and other peripheral passive elements. However, the active filter according JP-A 1-321,716 is a filter which has a summing function and a bandpass filtering function.
More specifically, the active filter according to JP-A 1-321,716 comprises a plurality of filter input resistors connected to a common node and the respective filter input terminals, an operational amplifier which has a predetermined gain and which has a positive input terminal, a negative input terminal connected to a reference voltage terminal, and an amplifier output terminal connected to the filter output terminal, a first amplifier input capacitor connected between the positive input terminal of the operational amplifier and the common node, a filter feedback resistor connected between the common node and the amplifier output terminal of the operational amplifier, and a second amplifier input capacitor and an amplifier input resistor which are connected between the positive input terminal of the operational amplifier and the reference voltage terminal in parallel.
It is therefore an object of the present invention to provide a low-pass filter with a summing function, which has a small scale or size in circuitry
It is another object of the present invention to provide a low-pass filter of the type described, which is capable of suppressing dissipation of an amount of power.
Other objects of this invention will become clear as the description proceeds.
According to this invention, a low-pass filter comprises first through N-th filter input terminals supplied with first through N-th filter input signals, respectively, where N represents a positive integer which is not less than two. Connected to the first through the N-th filter input terminals, summing and low-pass filtering means sums up the first through the N-th filter input signals to obtain a summed signal and removes a high frequency component from the summed signal to produce a low-pass filtered signal. Connected to the summing and low-pass filtering means, a filter output terminal produces the low-pass filtered signal as a filter output signal.
According to a first aspect of this invention, a low-pass filter has first through N-th filter input terminals supplied with first through N-th filter input signals, respectively, where N represents a positive integer which is not less than two, and a reference voltage terminal supplied with a reference voltage. The low-pass filter comprises an operational amplifier having an inverting input port, a noninverting input port, and an amplifier output port for producing an amplifier output signal. The noninverting input port is connected to the reference voltage terminal. An amplifier input resistor has an end connected to the inverting input port of the operational amplifier and another end connected to a common node. An amplifier input capacitor is connected between the reference voltage terminal and the common node. A feedback resistor is connected between the common node and the amplifier output port of the operational amplifier. A feedback capacitor is connected between the inverting input port of the operational amplifier and the amplifier output port of the operational amplifier. First through N-th input impedance circuits are connected between the common node and the first through the N-th filter input terminals, respectively. Connected to the amplifier output port of the operational amplifier, a filter output terminal produces the amplifier output signal as a filter output signal. Thereby, the low-pass filter sums up the first through the N-th filter input signals to obtain a summed signal and removes a high frequency component from the summed signal to make said filter output terminal produce a low-pass filtered signal as the filter output signal.
According to a second aspect of this invention, a low-pass filter has first through N-th positive filter input terminals supplied with first through N-th positive filter input signals, respectively, where N represents a first positive integer which is not less than two, and first through M-th negative filter input terminals supplied with first through M-th negative filter input signals, respectively, where M represents a second positive integer which is not less than two. The low-pass filter comprises an operational amplifier having inverting and noninverting input ports, and inverting and noninverting amplifier output ports for producing inverted and noninverted amplifier output signals, respectively. A first amplifier input resistor has an end connected to the noninverting input port of the operational amplifier and another end connected to a positive common node. A second amplifier input resistor has end connected to the inverting input port of the operational amplifier and another end connected to a negative common node. An amplifier input capacitor is connected between the positive and the negative common nodes. A first filter feedback resistor is connected between the positive common node and the inverting amplifier output port of the operational amplifier. A second filter feedback resistor is connected between the negative common node and the noninverting amplifier output port of the operational amplifier. A first filter feedback capacitor is connected between the noninverting input port of the operational amplifier and the inverting amplifier output port of the operational amplifier. A second filter feedback capacitor is connected between the inverting input port of the operational amplifier and the noninverting amplifier output port of the operational amplifier. First through N-th positive input impedance circuits are connected between the positive common node and the first through the N-th positive filter input terminals, respectively. First through M-th negative input impedance circuits are connected between the negative common node and the first through the M-th negative filter input terminals, respectively. Connected to the inverting amplifier output port of the operational amplifier, an inverting filter output terminal produces the inverted amplifier output signal as an inverted filter output signal. Connected to the noninverting amplifier output port of the operational amplifier, a noninverting filter output terminal produces the noninverted amplifier output signal as a noninverted filter output signal.